Linear polyolefins, in particular, linear polyethylenes, may be difficult to melt process. Specifically, due to a low shear sensitivity when compared to highly branched polyethylenes, the linear polyethylenes can require more extruder power to pump an equivalent amount of polymer melt. As a result, higher extruder head pressures, higher torque, greater motor loads, and the like can develop, as compared to the highly branched materials.
Increases such as higher motor load, head pressure and/or torque can place undesirable, unacceptable, or unattainable requirements on specific machinery. As for instance, a specific extruder having a specific motor power and gearing, will reach a maximum of motor load, or head pressure, under certain melt temperature conditions for a given polymer being processed. If a polymer is introduced to such an extruder which has such a higher requirement for power, such as a polymer having higher molecular weight and/or narrower molecular weight distribution and/or lower shear sensitivity, the extruder will reach a maximum of one or several of these parameters, and be therefore limited in its ability to pump/perform at a similar level to the performance expected/demonstrated with a highly branched or broader molecular weight distribution polymer such as traditional high pressure low density polyethylenes. In the alternative, if melt processing machinery is to be used for certain production/extrusion, and it is not so limited, the prospect of using more power or increasing head pressure for a more difficult to extrude material, while achievable, the user of the machinery would prefer to conserve power.
Additionally, linear polyethylenes may exhibit other imperfections during extrusion, specifically blown film extrusion, that may be undesirable, such as melt fracture. These imperfections are undesirable from a quality standpoint. For instance, melt fracture, also known as "shark skin" or "orange peel", can lead to poorer optical properties and/or diminished film physical properties, that are generally unacceptable.
The introduction of linear Ziegler-Natta catalyzed polyethylenes in the late '70s and early '80s and extruder owner's attempts to use these polyethylenes in machines that had been previously used to extrude free radical initiated, highly branched, high pressure produced low density polyethylenes provided the early manifestations of these problems. The advent of metallocene catalyzed linear polyethylenes in the '90s, has continued the trend towards polymers that when fabricated into films for instance, offer better physical properties and/or manufacturing economics, but have higher power requirements and/or greater tendency to exhibit melt fracture in the blown film process.
Linear polyethylenes therefore have been the subject of a good deal of effort to eliminate or reduce such problems. Some of the attempts included regearing extruders, designing new and more efficient screws and dies, increasing the power train, addition of expensive fluoroelastomeric processing aids and the like. In nearly every instance, the cost involved has not been inconsequential, as well as the inconvenience. But such costs have been born, due to the desirability of physical properties and/or downgaging possible with the linear polyethylenes.
GB 1,104,662 suggests addition of the salt of alkyl benzene sulfonic acids to polyolefins that purportedly gives a beneficial effect on melt extrusion behavior of the polyolefin. The purported effect is the reduction of the occurrence of "shark skin" or "orange peel". Both alkali and alkaline earth metal salts of alkyl benzene sulfonic acids are purported to be effective. The document is devoid of any identification of the polyethylene, such as molecular weight distribution (MWD), or composition distribution breadth index (CDBI).
GB 1,078,738 suggests that addition of an "external lubricant" to high molecular weight polyolefins can, purportedly, reduce occurrence of melt fracture. Suggested as external lubricants are salts of monovalent to tetravalent metals, and saturated or unsaturated carboxylic acids containing 10 to 50 carbon atoms. Sulfonates corresponding to the fatty acid salts are also said to be suitable. However, stearates, palmitates and oleates are exemplified. This document indicates an equivalence of metal salts of mono to tetra-valent metals.
JP A 59-176339 suggests that when polyolefins are narrowed in MWD or given higher molecular weight, poor fluidity results which in turn gives rise to melt fracture. The solution suggested is addition of fluorinated compounds including potassium salts of fluoroalkylsulfonic acids. These potassium salts are said to exhibit preferable temperature dependence when compared to other cations such as sodium, calcium, lithium and ammonium. The polyolefin/salt combination is said to be effective at 230.degree. C. or higher.
DE 2823507 suggests molding or calendered objects of ethylene polymers and propylene polymers containing alkalai or alkaline earth mono sulfonates from the group alkyl sulfonates, alkenyl sulfonates, alkylaryl sulfonates and succinic acid dialkyl ester sulfonates. Sodium or calcium mono sulfonates are preferred. A suggested benefit is purported to be outstanding separation of the polymer from calendering rolls.
JP 58-212429 (60-106846) suggests polyethylene compositions consisting of 70-95 weight parts of ethylene homopolymer or ethylene alpha-olefin copolymer with a density of at least 0.94 g/cm.sup.3 ; 5-30 weight parts of at least one of low density polyethylene (high pressure), ethylene vinyl acetate, ionomer, and ethylene alpha-olefin copolymer (density not exceeding 0.935 g/cm.sup.3); 0.01-5 weight parts of magnesium salt or calcium salt of alkylsulfonic acid or alkylbenzenesulfonic acid; and 0.05-0.5 weight parts of at least one substance selected from the group which includes dibenzylidene sorbitol or its nuclear substituted derivative. The combination is said to be especially useful in air-cooled inflation film.
U.S. Pat. No. 4,829,116 suggests polyolefin molding compositions purportedly having no surface defects that includes a fluorine-containing polymer together with a wax, preferred polyolefins are said to be ethylene copolymers with 1-olefins which contains 3-10 carbon atoms. The fluorine containing compounds are preferably copolymers of vinylidene fluoride and hexafluoropropylene or terpolymers of these monomers with tetra fluoroethylene. Among the suitable waxes enumerated are alkylsulfates or alkyl sulfonates containing straight chain or branched C.sub.8 to C.sub.26 alkyl radicals and an alkalai metal ion, preferably a sodium ion.
There is a need therefore for a relatively inexpensive, easily implemented solution to the processing problems outlined above. Such a solution should also include a material that when included in blown film extrusion of linear polyethylenes, will readily melt or incorporate into the melted polyethylene, and not adversely affect physical properties, be extractable, or negatively impact organoleptics of the film. Specifically, there is a commercial need for a material that may be easily incorporated into polyethylenes, that will reduce or eliminate the increased power requirement (e.g. motor load and or torque), increased head pressure, and melt fracture.